Latching relay with NE555
This was once one of my ideas for making true bypass for Boss and Ibanez style pedals. I've never used one of these as I intended, but the switching works well and could be used as a modification or basis for a clone build bypass scheme. And of course, this can be used in general purpose applications. I've used Zettler AZ850 five-volt relay. This relay has very wide coil voltage range (3,75 volts to 12,5 volts) and is suitable for many applications. This relay is monostable, non-latching type. This is important as we are making the latching part with NE555. Current consumption is about 25mA at five volts.
So, there are down sides to this. And one is definitely the current consumption. 25mA is not a lot itself, but if (for example) effects pedal takes only... let's say 5mA, it looks quite bad. For battery powered projects this is no go. And of course, when switching schemes becomes more complex the microcontroller is by far the best solution. No matter if the switch itself is a relay, transistor or something else.
For a simple on/off operation with one push button this is quick, simple, reliable and inexpensive solution. No code required, just pure hardware goodness. And you are not limited to the particular relay of this example. This design is easy to scale up just by changing different types of relays. You can switch very high currents with other relay types. These are two real world examples of this bypass system. Demeter Tremulator and Crowther Hot Cake (1977 version).